Progress in the digitization of protective relays has been promoting the application of a current differential protection principle, in which data communication is made by using a transmission path, to the power transmission line protection. In the current differential protection, a method for transmitting a sampled instantaneous value of an amount of electric current, has mainly been applied, but the method increases transmission capacity, as a result, it is required that a transmission path has, for example, a transmission rate of 1.544 Mbps. On the other hand, as a method to which a low speed general purpose transmission path is applicable, a current differential protective relay as disclosed in Patent Document 1, has been developed. The use of this method allows the application of a digital current differential protective relay that uses an existing pilot wire as a transmission path, and is promoting the replacement of existing analog pilot wire relays by digital current differential protective relays.
When a current differential protective relay is applied by using an existing pilot wire as a transmission path, since another system also uses an existing pilot wire as a transmission path, as shown in FIG. 7, there is a case in which protective relays 1 and other systems 2 (for example, remote monitoring and control system) share respective cores in one pilot wire cable used as a transmission path 3. Here, strictly speaking, the respective transmission paths 3 used for the protective relays 1 and the other systems 2 are different cores in the one pilot wire cable. Since in one pilot wire cable, a plurality of cores are very close to each other in position, the respective transmission paths 3 used for the protective relays 1 and the other systems 2 are substantially very close to each other in position. In such a case, it is required to consider, for example, adjustment of transmission level for the purpose of preventing the transmission of the protective relays 1 from interfering with the transmission of the other systems 2 on a transmission path to cause communication problems.
In a general transmission system, as an example of adjustment of transmission level, there are techniques as mentioned in Patent Document 2 and Patent Document 3.    Patent document 1: Japanese Patent Application Laid-open No. 2004-88920    Patent document 2: Japanese Patent Application Laid-open No. 2005-341022    Patent document 3: Japanese Patent Application Laid-open No. H05-30031 (No. 1993-30031)
As shown in FIG. 7, when protective relays 1 and other systems 2 such as monitoring and control system are configured by using cores in one pilot wire cable 3, it is required that the transmission level in transmission of the protective relays 1 is reduced to some extent so as not to cause communication problems in the other systems 2 under the influence of the transmission of the protective relays 1. However, since the conditions for transmission varies depending on the distance or the like of the pilot wire, it is impossible to fix in advance the transmission level.
As one method for adjusting the transmission level, the method of Patent Document 2 embeds a specific code into a signal to be transmitted to multiplex the signal, calculates attenuation in the transmission level from the transmission delay of the signal, the length of transmission line, and so on, and adjusts the transmission level. The method of Patent Document 2, when the duty value is low on the receiver side in optical communication, increases the transmission level and increases the margin of the received level. Both of these methods are directed to securing the transmission quality of the system itself, and not directed to preventing the influence on the other system when both systems use one pilot wire or the like.